®
ACT4065A
Rev 0, 23-Apr-12
FUNCTIONAL BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
As seen in,
Functional Block Diagram,
the AC-
T4065A is a current mode pulse width modulation
(PWM) converter. The converter operates as fol-
lows:
A switching cycle starts when the rising edge of the
Oscillator clock output causes the High-Side Power
Switch to turn on and the Low-Side Power Switch to
turn off. With the SW side of the inductor now con-
nected to IN, the inductor current ramps up to store
energy in its magnetic field. The inductor current
level is measured by the Current Sense Amplifier
and added to the Oscillator ramp signal. If the re-
sulting summation is higher than the COMP volt-
age, the output of the PWM Comparator goes high.
When this happens or when Oscillator clock output
goes low, the High-Side Power Switch turns off and
the Low-Side Power Switch turns on. At this point,
the SW side of the inductor swings to a diode volt-
age below ground, causing the inductor current to
decrease and magnetic energy to be transferred to
the output. This state continues until the cycle starts
again.
The High-Side Power Switch is driven by logic us-
ing the BS bootstrap pin as the positive rail. This pin
is charged to V
SW
+ 5V when the Low-Side Power
Switch turns on.
The COMP voltage is the integration of the error
between the FB input and the internal 0.808V refer-
ence. If FB is lower than the reference voltage,
COMP tends to go higher to increase current to the
output. Current limit happens when COMP reaches
its maximum clamp value of 2.0V.
The Oscillator normally switches at 210kHz. How-
ever, if the FB voltage is less than 0.6V, then the
switching frequency decreases until it reaches a
minimum of 30kHz at V
FB
= 0.15V.
Shutdown Control
The ACT4065A has an enable input EN for turning
the IC on or off. When EN is less than 0.7V, the IC
is in 8μA low current shutdown mode . When EN is
higher than 0.8V, the IC is in normal operation
mode. EN is internally pulled up with a 4μA current
source and can be left unconnected for always-on
operation. EN should never be directly connected to
IN.
Thermal Shutdown
The ACT4065A automatically turns off when its
junction temperature exceeds 155°C.
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TM
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